This invention relates to an improved process for the removal of acid gases, such as CO.sub.2 and H.sub.2 S, from hot, steam-containing gas mixtures.
The industrial importance of gas scrubbing processes involving the bulk removal of acid gases, particularly CO.sub.2 and H.sub.2 S, from various raw gas mixtures is steadily increasing. As the demand for synthetic fuels and synthesis gases derived from fuel sources such as natural gas, oil and coal increases, there is an ever-increasing need for efficient processes for the removal of CO.sub.2 and H.sub.2 S from the raw gas mixtures that are generated. For example, in the production of synthetic methane from naphtha, fuel oil or coal, the starting material is subjected to reforming or partial oxidation, producing a raw gas containing e.g. from 20% to as much as 50% of CO.sub.2 together with smaller amounts of H.sub.2 S where a sulfur-containing starting material is employed. Likewise, in the reforming of natural gas to produce hydrogen for ammonia synthesis or hydrogenation reactions, a raw gas is produced containing usually from 16% to 20% CO.sub.2, all of which must be removed prior to the ammonia synthesis step.
The acid gas-containing raw gas mixtures produced by such processes as steam-reforming and partial oxidation are at elevated temperatures (and usually at elevated pressures) and contain large amounts of steam. Good thermal efficiency demands the efficient recovery and utilization of the heat content of such raw gases. In this connection, the extent to which such heat content can be efficiently utilized to provide the energy required to remove the large quantities of acid gases they contain is a highly significant factor in determining overall energy efficiency.
In modern practice, the most widely used process for the bulk removal of CO.sub.2 and/H.sub.2 S from such gas mixtures involves scrubbing of the gas with aqueous alkaline scrubbing solution which is continuously recirculated between an absorption stage where the acid gases are absorbed and a regeneration stage in which the acid gases are desorbed from the solution by means of steam-stripping. For most applications, the most efficient type of such cyclic process utilizes a substantially isothermal absorption and regeneration cycle, i.e. the absorption and regeneration stages are operated at or close to the same temperature, viz. a temperature in the vicinity of the atmospheric boiling temperature of the scrubbing solution. By eliminating the heating and cooling that is required by non-isothermal processes, heat losses are greatly reduced.
In any such process, whether isothermal or non-isothermal, the major energy requirement in the process is the stripping steam for regenerating the solution, and it is accordingly highly desirable to reduce the regeneration heat requirements and/or derive such regeneration heat from heat sources that may have little or no utility for other purposes.
It is particularly desirable that maximum utilization be made of the heat content of the raw feed gas to provide the heat energy required to produce the necessary stripping steam and that such heat extracted from the raw process gas be at the lowest possible energy level so that the higher energy-level heat in the process gas can be utilized for other purposes. An efficient recovery of the low-level heat contained in the raw process gas will result in a lower-temperature feed gas to the absorption stage which, in turn, will improve the efficiency of the absorption stage.
There have been prior proposals for increasing the efficiency of heat recovery from hot, steam-containing gas mixtures which are to be treated for the removal of acid gases. See, for example, U.S. Pat. No. 3,823,222 to Homer E. Benson, issued July 9, 1974. In the process shown in that patent, the hot, steam-containing feed gas is passed in series through two heat exchangers, the first of which boils water to produce medium-pressure steam to operate a steam ejector, and the second of which raises relatively low-pressure steam in a heat exchanger where scrubbing solution is heated. The steam ejector is employed to raise additional low-pressure steam by subjecting regenerated scrubbing solution to a reduced pressure, and then to compress such low-pressure steam and inject it into the regenerator as additional stripping steam. The overall result is an enhancement in the net thermal efficiency of the process and a reduction in the amount of heat abstracted from the hot, steam-containing feed gas to produce the necessary stripping steam.